Prep two large aluminum cookie sheets with parchment paper and set aside.
Preheat oven to 410 degrees F (yep, that's right!)
In a stand mixer fitted with a paddle attachment, whip up the butter for 1 minute until it's light and fluffy, then add in the sugars to cream together with the butter for 3 minutes on high speed.
Scrape down the sides of the bowl and then add in the eggs, and vanilla. Whip up to combine.
Add in the flour, cornstarch, baking soda, and salt and slowly mix together by hand or on the very lowest mixer speed until just combined (don't overmix).
Add in the crushed cereal, regular cereal, white chocolate chips, and sprinkles. Mix in on the lowest-speed until just combined.
Split the dough into 8 balls (baseball-sized) and place 4 on each cookie sheet.
Bake for 9-10 minutes. They should be slightly golden, slightly mounded in the middle, thick/tall, and gooey in the center. When removed from the oven, immediately press on the whole lucky charms cereal marshmallows on top (sometimes I would pull off a bit of the cookie and placed the mallow cereal right in that tiny spot so that it sticks!). Let them cool on the pan for 5 minutes, then carefully transfer the cookies to a cooling rack to finish cooling. Enjoy!